Prepare playback immediately, new BackgroundTaskService.

- Immediately we start the CallDetailActivity with a voicemail, do the
  preparing of the media so we can see the duration.
- Use the duration text field to show "buffering..." until media source
  is prepared.
- Do the preparing in the background, this fixes a strict mode
  violation.
- If there's an error preparing, show an error message in that field.
- Add tests for both of the above cases.

BackgroundTaskService:
- Introduces new BackgroundTaskService, lightweight method for
  submitting AsyncTask objects.
- Introduces BackgroundTask interface and simple AbstractBackgroundTask
  abstract implementation.
- Adds BackgroundTaskService to ContactsApplication allowing Activity
  objects to get hold of a regular background task service.
- Adds a MockBackgroundTaskService for use with injecting for test, so
  that we can prevent or control processing of background tasks.

Other:
- Every time we resume the Activity, we were causing a new voicemail
  fragment to be created.  There was no bug tracking this, I just
  noticed it because of occasionally flaky tests.  Added a fix and test
  to catch it again next time.
- Fixes missing tear down method in PeopleActivityTest.

Bug: 5115133
Bug: 5059965
Bug: 5114261
Bug: 5113695
Change-Id: Ia2469229fa756da8b3977231fbf23a9d3fb379ce